1. FIELD OF THE INVENTION
The invention relates to the preparation of aqueous dispersions containing silica produced by pyrogenic means. A process for the preparation and the use of the dispersions for polishing semi-conductor substrates is included in the invention.
2. DISCUSSION OF THE RELATED ART
Aqueous dispersions containing silica have a broad field of application. The applications include, for example, the coating of paper, the manufacture of glass fibers and quartz glass, and the chemical-mechanical polishing of semi-conductor substrates (CMP process).
Conventional dispersions are based either on colloidal silica, silica sols, or silica produced by pyrogenic means.
Colloidal silica is produced from solutions of sodium silicate and yields dispersions with a very small particle size and very good dispersion stability. A disadvantage, particularly when polishing semi-conductor substrates, is the amount of impurities introduced by the starting material sodium silicate, and the adhesion of the particles to polished surfaces.
Pyrogenic silica, on the other hand, produced by flame oxidation or flame hydrolysis from silicon tetrachloride, hydrogen and oxygen, exhibits a very high purity and a primary particle size comparable with that of colloidal silica. The primary particles aggregate and agglomerate, however, producing hard particles. Dispersion of the aggregates and agglomerates proves to be difficult, the dispersions are less stable and are susceptible to sedimentation or gelling.
A possibility of increasing the stability of the dispersion is described in U.S. Pat. Nos. 5,116,535 and 5,246,624. A particular feature is the BET surface which should be no greater than 75 m2/g, preferably from 30 to 60 m2/g. Here, too, however, as described in U.S. Pat. No. 5,904,159, only a slightly better stability can be expected. Sedimentation occurs after only two months.
U.S. Pat. No. 5,904,159 describes an aqueous dispersion containing silica produced by pyrogenic means which has improved stability. This is achieved by a particular form of dispersion process. The use of a high-pressure homogenizer permits the preparation of aqueous silica dispersions with average particle diameters from 30 nm to 100 nm which are claimed to be stable for several months without sedimentation occurring.
A similar form of dispersion process is also described in EP-A-876 841. The average particle size of the claimed metal oxides produced by pyrogenic means is given here as 10 nm to 2 xcexcm and the stability is given in the examples as at least 30 days.
This dispersion process its also claimed in WO 00/17282 Al for silica, cerium oxide and zirconium oxide, with particle sizes from 30 nm to 500 nm. No indications about the stability of the dispersion are given.
Despite this improved stability of the aqueous dispersions, the reagglomeration behavior of the dispersed silica particles remains, limiting the stability of the dispersion and leading to scratches when surfaces are polished in the CMP process.
The object of the present invention is to provide an aqueous dispersion which contains silica produced by pyrogenic means which exhibits a markedly reduced reagglomeration behavior, greater stability and which, during chemical-mechanical polishing, yields high removal rates and leads to a surface which is very largely free from microscratches.
It is a further object of the invention to provide a process of preparing the silica dispersions of the invention.
It is another object of the invention to utilize the silica dispersions of the invention for planarizing semiconductor surfaces or as coatings or fillers.
The technical object is achieved by an aqueous dispersion containing an alkali-doped pyrogenic silica prepared by means of aerosol, where the dispersion contains silica having an average particle diameter of secondary particles of less than 100 nm and the quotient dn/da of the arithmetic mean of the number distribution dn and the arithmetic mean of the surface distribution da of the primary particles is at least 0.7.
The terms primary and secondary particles originate from the pyrogenic production of the alkali-doped silica. In a pyrogenic process for the production of doped and undoped silica, the so-called primary particles initially have a size which is dependent on the reaction parameters selected and which is approximately from 5 nm to 40 nm. The size of the primary particles may be determined, for example, by TEM pictures. The primary particles are not, however, present in the isolated form but intergrow to aggregates, or join together to form agglomerates which are described hereinafter as secondary particles.
The quotient dn/da in the alkali-doped silicas of at least 0.7 describes a markedly narrower particle size distribution than in an undoped silica produced by pyrogenic means.
The quotient dn/da in a silica produced by pyrogenic means with a BET surface of 130 m2/g (Aerosil 130, Degussa AG) is 0.52.
This dispersion has greater stability than dispersions prepared without alkali oxide doping. Greater stability of the doped dispersion means that the time at which the dispersion increases in viscosity, geling or settling of the silica occurs later than in dispersions with undoped silica.
It also became apparent that the time for an even distribution of the alkali-doped silica produced by pyrogenic means is shorter than without doping.
This result is surprising because aqueous dispersions containing silica produced by pyrogenic means may be stabilized anyway, according to the prior art, by the addition of KOH or another basic substance.
Potassium-doped silica has a modified aggregate or agglomerate structure compared with undoped silica. The doping substance is homogeneously incorporated, in the pyrogenically prepared alkali-doped silicas of the invention. In contrast to mixtures of undoped silica and alkali, where the alkali is present on the exterior of the silica particles, the alkali-doped silica particles of the invention contain alkali both inside and on the exterior of the particles. For this reason, a dispersion which contains alkali-doped silica and alkali differs clearly from one which contains undoped silica and alkali. In the case of alkali-doped silica, this different structure leads to more rapid incorporation, a lower reagglomeration tendency, and hence to greater stability of the aqueous dispersions.
In the case of silica produced by pyrogenic means and doped by means of aerosol, the doping component is fed into the flame, of the kind used in the known way for the production of pyrogenic oxides by flame hydrolysis, in the form of an aerosol, for example, in the form of an aqueous solution of alkali chlorides. This process is described in DE-A-196 50 500 (incorporated herein by reference). The aerosol is mixed homogeneously prior to the reaction with the gas mixture of flame oxidation or flame hydrolysis, silicon tetrachloride, hydrogen and oxygen. The aerosol-gas mixture is allowed to react in a flame and the resulting doped silica produced by pyrogenic means is separated from the gas stream in the known way. The starting product of the aerosol is a salt solution or suspension containing the component of the doping substance. During the formation of the pyrogenic oxide, the doping medium is in the fine-particle form in the gas phase, so that homogeneous incorporation of the doping component in the silica produced by pyrogenic means is possible. Consequently, the aggregate or agglomerate structure of the pyrogenic silica is also influenced.
The degree of doping may be varied widely in the silica prepared by the above process, from 0.00001 wt. % to 20 wt. %. In the use for the preparation of an aqueous dispersion according to the present invention, the degree of doping is preferably from 10 ppm to 10 wt. %, particularly preferably in the range from 300 ppm to 2 wt. %.
The BET surface of the alkali-doped silica in a preferred embodiment of the invention is from 5 to 600 m2/g. A range from 50 to 400 m2/g is particularly preferred, in which the dispersion exhibits good stability and the preparation of the alkali-doped silica is technically simple to carry out.
The solids content of the dispersion containing alkali-doped silica depends primarily on the intended use. In order to save on transport costs a dispersion with the highest possible solids content is desirable, whereas for certain applications, such as, for example, for polishing silicon wafers, dispersions with low solids contents are used. The range from 0.1 wt. % to 70 wt. % is preferred according to the invention, the range from 1 wt. % to 30 wt. % being particularly preferred. In these ranges, the alkali-doped dispersion shows good stability.
Due to the fact that the silica is doped with alkali, the pH of the dispersion is higher than in an undoped one, from about 5 to 8 depending on the degree of doping (pH of a four percent dispersion). The dispersion may be used such as it is, for example, for polishing. As in the case of dispersions containing undoped silica, however, the viscosity in this case is markedly increased in a pH range from acid to slightly alkaline. According to a preferred embodiment of the invention, the pH of the dispersion is adjusted to a value from 8 to 12 by adding alkali hydroxides or amines, potassium hydroxide and ammonia or ammonium hydroxide being particularly preferred. This leads to a marked stabilization of the dispersion, and condensation reactions of the silica are thereby avoided.
According to DE-A-196 50 500, all alkali metals are suitable for doping silica produced by flame oxidation or flame hydrolysis. Doping with potassium is, however, particularly preferred. When potassium salts are used as the doping component, the structure alters decisively, that is, the degree of intergrowth and also the morphology (that is, the appearance) of the primary particles. In potassium doped silica, this change in morphology starts at a potassium content of more than 300 ppm.
Surprisingly, the pyrogenic oxides doped in this way with potassium exhibit spherical round primary particles with only very little intergrowth in the electron micrograph, which also manifests itself in the fact that no end point is detectable when the structure is determined by the dibutyl phthalate method (DBH method). The potassium is uniformly distributed in the doped pyrogenic oxides. This cannot be seen on the electron micrographs.
FIG. 1 shows an electron micrograph of a silica produced by pyrogenic means without doping (Aerosil 130, Degussa).
FIG. 2 shows an electron micrograph of a silica produced by pyrogenic means and doped with 0.44 wt. % potassium, with a specific surface (BET) of 131 g/m2.
The invention also provides a process for the preparation of the dispersion containing alkali-doped silica. Dispersion methods suitable for this purpose are those in which a sufficiently high energy input permits dispersion of even very hard and highly aggregated materials. These include systems operating on the rotor-stator principle, fox example, Ultra-Turrax machines or agitated ball mills. Higher energy input is possible with a planetary kneader/mixer. The effectiveness of this system is associated, however, with a sufficiently high viscosity of the treated mixture in order to introduce the requisite high shear energies for breaking down the particles.
When doped oxide particles are ground and dispersed, there is a risk that the dopant will become detached during grinding and dispersion. If the dispersion is to be used in the CMP process as a polishing agent, this leads to uneven polishing results.
It has now been found that aqueous dispersions containing alkali-doped silica particles which are smaller than 100 nm and in which the dopant does not become detached may be obtained with high-pressure homogenizers, hereinafter also called wet-jet-mill.
In these devices, two pre-dispersed streams of suspension under a pressure of up to 3500 kg/cm2 are depressurized by means of a nozzle. Both dispersion jets strike each other exactly and the particles grind themselves. In another embodiment, the pre-dispersion is likewise placed under high pressure, but the collision of the particles takes place against armour-plated wall regions.
These devices have been used hitherto only for the dispersion of undoped oxides such as zinc oxide, silica, aluminium oxide (UK-A-2 063 695, EPA-876 841, EP-A-773 270, WO 00/17282 Al). The grinding and dispersion of doped oxides with these devices has not been described.
The invention also provides the use of the aqueous dispersion of alkali-doped silica for planarizing semiconductor substrates or layers applied thereto. A microscratch-free surface may be obtained with the dispersion of alkali-doped silica with a high rate of polishing. Moreover, said dispersions are suitable for the preparation of very fine-particle surface coatings in the paper sector, or as a raw material in the cosmetics and glass sector.
German application 10065 027.9, filed on Dec. 23, 2000 is incorporated herein by reference.
Where ranges are provided herein, all values and sub-ranges between and including the stated values are included.